1. Field of the Invention
The subject invention relates to a light emitting diode assembly, and more specifically, to thermal management for conducting thermal energy away from the diode.
2. Description of the Prior Art
Assemblies in the prior art include a light emitting diode (LED) with first and second electrical leads for conducting electricity to and from said light emitting diode, and a heat sink. For several decades the preferred light emitting diode construction was the so-called T13/4 epoxy package. This inexpensive package is more than adequate at relatively low LED power levels. As LED performance levels rose, and the power dissipated within the devices reached a critical level and the self-generated heat within the LED die itself became an important design issue. The well-known behavior of many LED families to substantially dim and degrade at higher operating temperatures drove the need for better thermal management solutions.
Larger LED dies and more efficient, low thermal resistance leads or lead frames filled much of the need for higher performance devices. However, as luminous
output increased by substantially increasing device drive current, self-generated heat again became a design issue.
Various dual in-line LED packages provided decent luminous flux at nominal cost for all but the most demanding applications. Unfortunately, it is these more demanding applications that offer the greatest market potential. Outdoor lighting of various kinds, such as automotive exterior lamps, traffic signals, railroad signals, and even advertising signs, are exposed to high ambient operating temperatures. When coupled with the self-generated heat of the LED itself, the resulting die (junction) temperature may quickly degrade the LED, shortening it""s life and reducing it""s light output. For some safety critical applications such a reduction in luminous output can have dire consequences.
In order to ameliorate these thermally driven problems, LED manufacturers began to manufacture more, thermally capable devices. One such device is designed only for mechanical crimp attachment, as the relatively low thermal resistance of its lead frame may damage the LED die if the device is soldered. Another path to high performance LEDs with aggressive thermal management is exemplified by products which have essentially separated the major heat flow path out of the die from the electrical leads that power the devices. One assembly employs an elegant yet costly bulk diamond insulator to de-couple the die electrically from the integral heat sink post. This plated copper element transfers heat from the die to an external heat dissipater. The use of a bulk (or thin film) diamond insulator is advantageous because of diamond""s very high thermal conduction, which is greater than that of copper (400 W/m/xe2x80x3K). Unfortunately, the excellent thermal performance comes at a high price, and such LEDs are typically priced at least an order of magnitude above less sophisticated but nearly comparable xe2x80x98non diamondxe2x80x99 LEDs. Another assembly modifies the existing commercial dual-in-line package to accommodate an Integral heat sink but wherein an electrically non-conductive material must be placed between the diode and the heat sink to make sure no electrical current passes from the diode to the heat sink, e.g., U.S. Pat. No. 5,857,767. While the thermal impedance of this LED is nominally 3.7 times higher than the design with diamond insulation, cost/performance criteria favor this design.
There clearly remains a need for an improvement and modification of the current designs to improve the thermal performance of a non-diamond isolated LED to a level comparable to the costly diamond isolated LED.
An electrically driven light emitting diode (LED) assembly comprising a light emitting diode, first and second electrical leads for conducting electricity to and from said light emitting diode, and a heat sink. The assembly is characterized by the first lead including the heat sink for conducting electricity and heat from the light emitting diode through the heat sink.
Also included in the invention is a method of fabricating such an electrically driven light emitting diode (LED) assembly comprising the step of disposing the heat sink in electrical series with the first lead for conducting electricity and heat from the light emitting diode through the heat sink.
Accordingly, the present invention comprises an LED assembly that is constructed with an integral primary heat flow path other than the electrical leads. That heat flow heat sink may be electrically insulated on the outside of the LED when electrical isolation is required by coupling to an electrically conductive heat dissipater.